The recorded quality and cross-compatibility of optical media, such as optical discs, can be affected by the optical power used for writing data onto the optical media. Additionally, the optimum power level for writing optical media data can depend on many factors, such as media type, modulation criterion, writing speed, drive and type of hardware used. Also, the optimum power level can differ from system to system due to component and media variations. Thus, it can be difficult to determine the optimum power without the application of so-called Optimum Power Control (OPC) methods.
Standard OPC methods typically consist of first writing several short optical media data test segments. The data test segments are written with different power levels in a region of the optical media dedicated to this test procedure. Writing the data test segments may also involve the selection and application of a particular modulation criterion. The modulation criterion may be selected, for each implementation of an OPC method, from modulation criteria indicated as available for application in media specifications associated with each type of optical media.
Subsequently the data test segments are read back from the optical media and an analysis of the signal's modulation properties is performed. This analysis enables an optimum power level to be established. The collective execution of these stages of writing, reading, and analysis, which enables the determination of the optimum power level, is sometimes referred to herein as an optimal power control (OPC) calculation.
During an OPC calculation, it is impractical to test the entire range of power values possible at high resolution, due to limits on available processing time and required optical media area. Consequently, the testing is typically constrained to a limited range of values. In some optical media drive implementations, the test range is determined using a calibration procedure performed at the time of manufacture. In other cases, the test range is determined based upon an expected variation of the components and optical media used.
However, due at least in part to variations in the optical media drive and optical media performance, the appropriate power range for an OPC calculation may vary with component aging, and also may not be cross-media compatible. The invalidity of a signal modulation may also depend on the applied modulation criterion itself; for example, it may be too low to read, too high (saturated), or not in the correct area for linear approximation. Furthermore, for cases where offline power calibration is not applied to a system, an accurate power level may not be achieved during the writing stage of an OPC calculation, increasing the undesired bias of the results, since the power range intended for use may be different from that which is measured. As a result, modulation information read back may be only partially valid, and may degrade the OPC calculation, or even cause it to fail.